Ticket/11305 unbreak installer after cache service gained dependency on db connection

http://tracker.phpbb.com/browse/PHPBB3-11305

This patch does 3 things:

1. Moves obtain_hooks from cache service into a new class, hook finder.
2. Deletes instantiation of cache service as part of installer bootstrap.
3. However, final installation step still needs a cache service. In that step the installer container is replaced by a normal container and cache service is instantiated.

The third bullet point should have a hidden bonus of creating compiled container in installer.